1. Field of the Invention
The present invention relates to overcurrent protective devices for power devices.
2. Description of the Background Art
FIG. 4 is a circuit diagram of a conventional drive and overcurrent protective device for power devices. A drive signal S2 at H/L level is given from a drive circuit 2 such as a driver to the gate of an N-channel IGBT (insulated gate bipolar transistor) 1 to control the on/off operation of the IGBT 1.
The IGBT 1 is provided with a sense terminal 3 separately. The sense terminal 3 is grounded through a sense resistor 4. Since the sense terminal 3 is adapted to carry a current proportional to a collector current IC of the IGBT 1, the current proportional to the collector current IC flows in the sense resistor 4. As a result, the level of a sense voltage VS provided from the sense terminal 3 is proportional to the collector current IC.
The sense terminal 3 of the IGBT 1 is connected to the positive inputs of comparators 51 and 52 in common. A reference voltage VR1 is applied to the negative input of the comparator 51, and a reference voltage VR2 (&lt;VR1) is applied to the negative input of the comparator 52.
An output signal S51 of the comparator 51 is applied to the input of an NOR gate 6. An output signal S52 of the comparator 52 is applied to a signal judging circuit 7. A judgment signal S7 of the signal judging circuit 7 is applied to the input of the NOR gate 6.
The signal judging circuit 7 normally outputs the L-level judgment signal S7 and outputs the H-level judgment signal S7 when a period that the output signal S52 of the comparator 52 to be inputted thereto is at the H level is not shorter than an allowable duration At.
A control signal SC is also applied to the input of the NOR gate 6. An output signal S6 of the NOR gate 6 is applied to the input of the drive circuit 2.
In such arrangement, as the control signal SC that is set at the L level is inputted to the NOR gate 6, the output signal S6 of the NOR gate 6 turns to the H level since the two other input signals S51 and S7 of the NOR gate 6 are normally at the L level. As a result, the drive circuit 2 gives the H-level drive signal S2 to the gate of the IGBT 1, so that the IGBT 1 turn on.
The sense voltage VS is equal to or lower than the reference voltage VR2 with respect to the collector current IC of the IGBT 1 in normal operation. The output signals S51 and S52 of the comparators 51 and 52 are held at the L level, so that the H level of the output signal S6 of the NOR gate 6 is unchanged. The IGBT 1 is held "on".
As the collector current IC of the IGBT 1 grows high due to increase in load and the like, the sense voltage VS accordingly grows high. When the sense voltage VS exceeds the reference voltage VR2, the output signal S52 of the comparator 52 turns to the H level.
However, the judgment signal S7 of the signal judging circuit 7 is held at the L level when the period of VS&gt;VR2 is shorter than the allowable duration .DELTA.t, so that the IGBT 1 is held "on". When the period of VS&gt;VR2 is equal to or longer than the allowable duration .DELTA.t, the judgment signal S7 of the signal judging circuit 7 turns to the H level. Then the output signal S6 of the NOR gate 6 turns to the L level and the L-level drive signal S2 is given from the drive circuit 2 to the gate of the IGBT 1. Accordingly, the IGBT 1 turns off and the collector current flowing in the IGBT 1 is interrupted to permit overcurrent protection to function.
As the collector current IC further grows high until the sense voltage VS exceeds the reference voltage VR1, the output signal S51 of the comparator 51 turns to the H level, so that the output signal S6 of the NOR gate 6 instantaneously turns to the L level independently of the H/L level of the judgment signal S7 of the signal judging circuit 7. As a result, the L-level drive signal S2 is given from the drive circuit 2 to the gate of the IGBT 1. Accordingly, the IGBT 1 turns off and the collector current flowing in the IGBT 1 is interrupted to permit the overcurrent protection to function.
As above described, the conventional overcurrent protective device for the IGBT, on detecting an overcurrent state in the IGBT by detecting the collector current of the IGBT, causes the IGBT to turn off whereby the overcurrent protection functions.
FIG. 5 is a graph showing an operating area of the IGBT. In FIG. 5, IC1 represents the collector current where the sense voltage VS is equal to the reference voltage VR1, and IC2 is the collector current where the sense voltage VS is equal to the reference voltage VR2. A curve L1 indicates the upper limit of the safe operation of the IGBT. That is, the safe operating area of the IGBT is an area in which the collector current IC varying with time is below the maximum safe operation curve L1. The IGBT is enabled to operate safely within the safe operating area.
Unfortunately, the drive and overcurrent protective device for the IGBT shown in FIG. 4 determines only shaded regions B1 and B2 of FIG. 5 as the safe operating area. There has been a problem in that the collector current IC varying with time, if practically within a region A1 included in the safe operating area, is taken as being in an abnormal operating area to cause the protective function to operate.
Furthermore, fixed are the reference voltages VR1 and VR2 serving as a reference for comparison of the comparators 51 and 52 and the allowable duration .DELTA.t by the signal judging circuit 7. Another problem is that it is impossible to judge the safe operating area adapted for the practical IGBT safe operating area varied with temperature changes of the IGBT itself.